K-Ar ages of some volcanic rocks from the Cook and Austral Islands G. BRENT DALRYMPLE U.S. Geological Survey, Menlo Park, California 94025 R. D. JARRARD* Marine Physical Laboratory, Scripps Institution of Oceanography, La Jolla, California 92037 D. A. CLAGUE* Geological Research Division, Scripps Institution of Oceanography, La Jolla, California 92037 ABSTRACT northwest and tend to be youngest at the ing of volcanoes in the chain seemed neces- east-southeast end. He suggested that these sary, both to confirm the reliability of pre- K-Ar age measurements on 19 volcanic chains were formed by relative motion be- vious K-Ar dating and to provide a stronger rocks from Rurutu, Mangaia, Rarotonga, tween the lithosphere and a hot spot (or test of the general melting-spot hypothesis. and Aitutaki in the Cook-Austral chain do melting spot) in the mantle. Morgan (1972) This paper presents the results of some ad- not show a systematic increase in the age of offered the hypothesis that these hot spots ditional K-Ar measurements and their im- the volcanoes to the west-northwest away were fixed, both relative to one another and plications for the origin of the Cook- from Macdonald Seamount as predicted by to the Earth's spin axis, and that they were Austral volcanic chain. the melting-spot hypothesis and suggested caused by mantle convection in the form of Age measurements were by the conven- by geomorphic evidence. Ages determined narrow plumes that brought material up- tional K-Ar method with the use of previ- for alkalic basalt samples from Rurutu ward from near the core-mantle boundary. ously described techniques (Dalrymple and range from 1.02 to 1.09 m.y., for Mangaia Other mechanisms proposed for the origin Lanphere, 1969; Ingamells, 1970). Ar from 16.6 to 18.9 m.y., and for Aitutaki of linear island chains include propagating analyses were by isotope-dilution mass from 0.66 to 0.77 m.y. Two distinct periods fractures (Betz and Hess, 1942; Green, spectrometry; K was measured using of volcanism on Rarotonga were dated at 1971; Jackson and Wright, 1970; Vogt, lithium metaborate fusion and flame 1.8 and 1.2 m.y. B.P. The relation between 1974), diapiric upwelling (McDougall, photometry. the dated units and the main shield-building 1971), and melting caused by shear beneath stage of these volcanoes is uncertain and the lithosphere (Shaw, 1973; Shaw and PETROGRAPHY AND will remain so until better data on the erup- Jackson, 1973). Morgan (1972) suggested SAMPLE LOCATIONS tion history and mode of formation of vol- that the Austral-Southern Cook-Gilbert- canoes in the chain are available. Key Marshall band of islands and seamounts re- Thin sections of 46 samples from Rurutu, words: geochronology, potassium-argon, sulted from motion of the Pacific plate over Mangaia, Rarotonga, and Aitutaki were Cook Islands, Austral Islands, volcanic a melting spot now located at Macdonald examined to determine their suitability for chain, melting spot, Pacific plate, petrog- Seamount. K-Ar dating according to criteria discussed raphy, volcanology, basalt. Regardless of the exact dynamic by Mankinen and Dalrymple (1972). For mechanism proposed for melting spots, a Rarotonga, from which numerous speci- INTRODUCTION corollary of the general kinematic mens were available, samples were chosen hypothesis is that the ages of islands and to represent a broad range of rock types The Austral and Southern Cook Islands seamounts in each chain generally increase and the several stratigraphic units exposed are part of a quasi-linear volcanic chain of away from the present location of the melt- on the island (Wood and Hay, 1970). A seamounts and islands in the South Pacific ing spot. The ages of individual volcanoes table1 describing the general petrographic Ocean (Fig. 1); the chain extends west- within a chain thus provide an important features of the dated samples is available northwest for more than 2,000 km from test of the general melting-spot hypothesis. from The Geological Society of America Macdonald Seamount, which is thought to K-Ar dating of volcanoes in the Hawaiian- depository. Rock names used in Table 1 are be currently active (Norris and Johnson, Emperor chain has confirmed an increase in after the classification of Williams and 1969; Johnson, 1970). Some of the vol- age away from the active volcano of others (1954). All samples we dated were canic islands in the Austral-Cook chain Kilauea, although the increase with dis- collected by A.E.J. Engel. Unfortunately, were described by Marshall half a century tance is only approximately linear the field notes describing the location of ago (1908, 1909, 1912, 1927, 1929, 1930) (McDougall, 1964; Jackson and others, these samples were destroyed, and although and by Chubb (1927a, 1927b). More re- 1972; Dalrymple and others, 1974; Clague the island from which each sample was col- cently, Wood (1967) and Wood and Hay and others, 1975). Relative motion between lected is known, the exact location and (1970) described the geology and petrology two melting spots can be detected by com- stratigraphic position for any of the sam- of the Cook Islands. paring the age-distance relation of two dif- ples on individual islands are not known The origin of linear volcanic chains is of ferent chains that have formed on the same with certainty. No ages were determined considerable current interest. Wilson lithospheric plate. The few available for Rapa, although suitable samples were (1963a, 1963b) recognized that several radiometric (Krummenacher and Noetzlin, available from the collections of L. J. chains in the Pacific trend generally west- 1966; Tarling, 1967) and paleontologic Chubb (Smith and Chubb, 1927) and (Marshall, 1927, 1930; Wood and Hay, 1970) ages for the Southern Cook and Aus- * Present address: (Jarrard) Department of Geology, tral Islands were only partly consistent 1 Copies of GSA supplementary material 75-29 may University of California, Santa Barbara, California be ordered from Documents Secretary, Geological Soci- 93106; (Clague) U.S. Geological Survey, Menlo Park, with the fixed-melting-spot hypothesis ety of America, 3300 Penrose Place, Boulder, Colorado California 94025. (Clague and Jarrard, 1973a). Further dat- 80301, USA. Geological Society of America Bulletin, v. 86, p. 1463-1467, 1 fig., October 1975, Doc. no. 51018. 1463 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/10/1463/3418525/i0016-7606-86-10-1463.pdf by guest on 25 September 2021 1464 DALRYMPLE AND OTHERS A.E.J. Engel, because Krummenacher and Group is overlain by rocks of the Muri date both the Te Manga Group and the Av- Noetzlin (1966) had obtained concordant Flows and the Te Kou Complex, both of the atiu Caldera Complex. The ages reported ages of 5.1 ± 0.4, 5.0 ± 0.2, and 5.2 ± 1.7 extracaldera group. The nepheline phono- by Tarling not only disagree with our m.y. on samples from the island. Five sam- lite (PV575) is identical to one described by measurements but are stratigraphically in- ples collected from Atiu by Peter Wood Marshall (1908) from Muri Point and is consistent, and we conclude that they are were examined but were not dated because probably from the Muri Flows. The anor- probably incorrect. of alteration or high carbonate content. thoclase trachyte (PV504) might be from The six samples from Mangaia range in The main mass of Rurutu is alkalic either the Te Kou Complex, the Muri age from 16.6 to 18.9 m.y. The difference basalt. The island rises to an elevation of Flows, or the Maungatea Breccia, where between the oldest and the youngest calcu- about 400 m and is bounded by an elevated Wood and Hay (1970) described anortho- lated ages is statistically significant at the 95 limestone reef or "makatea" approximately clase trachyte units. percent level of confidence, and the appar- 75 to 100 m above sea level and by a The lava flows exposed on Aitutaki are ent range of ages may represent a real dif- present-day fringing reef. The dated sam- primarily nepheline basalt and associated ference between the ages of the flows sam- ples from Rurutu are similar to Smith and mafic alkalic basalt, although on Rapota pled. Chubb's (1927) nepheline tephrite sample (an islet that is part of Aitutaki) they are Though the samples are unlocated, the 797 but are here called oligoclase basalt be- overlain by an agglomerate that contains relation between the dated samples and the cause nepheline was not found in thin sec- pebbles of trachyte and phonolite that were exposed lava flows of each volcano can be tion. Four samples collected in 1925 by possibly erupted during a late stage in the inferred. It appears that the oldest exposed Chubb were examined through the courtesy volcanic history of the island (Wood and parts of Mangaia and Rarotonga have been of the British Museum of Natural History, Hay, 1970). The stratigraphically oldest dated. For Aitutaki, the relation is less cer- but none was suitable for dating. flows exposed on the island are of nepheline tain. Wood and Hay (1970) observed that Mangaia Island is formed of low, deeply basalt; they are underlain in places by Maungapu (the inferred location of our eroded central hills of basalt surrounded by palagonitic ash and weathered basalt Aitutaki samples) is on the western edge of a makatea. Although the flows are poorly scoria. The dated nephelinite samples are a craterlike feature that may have de- exposed, they appear to represent a single identical to those described by Wood and veloped during a rejuvenation of volcanic eruptive series, unbroken by any erosional Hay (1970) from near Maungapu (a prom- activity. Our samples, however, do not re- hiatus.